In various digital systems, signals can be transmitted from a transmitter to a receiver via a transmission channel. The transmission channel may be any suitable wired (or wireless) medium which links the transmitter to the receiver. However, in many instances (e.g., high data transmission speeds), the transmission channel becomes lossy. The transmission losses can be a result of, among other things, interference, attenuation, and delay in the channel. Further, such losses can also have considerable detrimental effect on the transmitted signal by the time it reaches the receiver. For example, sufficient amplitude and phase distortion of the transmitted signal may result in intersymbol interference (ISI) in the signal received at the receiver. ISI generally refers to the ‘smearing’ of a pulse or other symbol representing the logic state of one data bit to the degree such that it contributes to the content of one or more of the preceding (i.e., pre-cursor ISI) or succeeding (i.e., post-cursor ISI) data bits.
To guard against such detrimental effects, many serial receiver systems perform decision feedback equalization (DFE) on the received data. Such serial receiver systems may include (i) an analog front end that provides some continuous time linear equalization (CTLE), (ii) a sampler, a (iii) DFE that uses the quantized receive data to adaptively feedback a correction signal, and (iv) a timing recovery unit. The timing recovery unit may use edge samples of the signal to determine if the received timing is early or late (i.e., phase detection). This information may go to a digital loop filter, which outputs to a phase selector in order to generate a recovered clock. This recovered clock may then be used to sample (i.e., with the sampler) the input signal and process the received data. However, in certain instances, the phase detection may also be corrupted by the DFE correction. For example, if a large number of the edge locations that the timing recovery unit is locking to are predominately affected by the DFE feedback, the timing recovery unit will lock to the DFE feedback instead of the incoming signal. Further, because the timing of this DFE feedback is generated by the timing recovery itself, when the DFE feedback signal strength becomes significant relative to the incoming data, the timing recovery will diverge from the ideal sampling phase and frequency. Similarly, the timing recovery can also fail to lock (or lose lock) if, in addition to a strong DFE feedback, the transmission channel has high loss and dispersion, and the incoming signal is weak.
Previous solutions have used two methods to address this interaction between the timing recovery and the DFE feedback: (i) splitting the clock and data recovery of the timing recovery unit into two paths (i.e., one clock path and one data recovery path) and (ii) using a pattern filter to remove edges from the timing recovery unit that can be corrupted by the DFE. As regards to the first solution, the clock path does not include the DFE feedback, thereby avoiding this interaction. Further, a phase recovery block is used to align the two paths and to compensate for any slow drifts in the delay of the two paths. Unfortunately, the dual path architecture adds extra complexity to the circuitry and the phase recovery used to sample the data path must also have some mechanism to prevent interaction with the DFE. As regards to the second solution, the pattern filter removes the edges based on previous data bits. Although many of the edges of the incoming signals can be corrupted by the DFE feedback, some are not. Therefore, if pattern filtering is applied to selecting only those edges that are not corrupted by the DFE feedback, the above-discussed interaction problem can be avoided. However, filtering out these edges also reduces the updates into timing recovery, thereby lowering the bandwidth. Further, filtering also requires certain data sequences be avoided, i.e., those data sequences that predominately consist of a clock pattern (e.g., 010101 . . . ). Therefore, if the pattern filter only uses the previous data bits, these undesirable effects (e.g., lowered bandwidth and avoidance of certain data sequences) will persist even in the case where the transmission channel has low loss and the interaction is small enough such that the pattern filter was likely not needed.
Accordingly, there is a need for adaptively applying the pattern filters so that the edges are discarded only when the DFE feedback has adapted to levels that can corrupt the timing recovery.